Heat-sensitive recording sheets are used, for example, in facsimile receivers and printers. Such sheets contain, as main components, a heat-sensitive layer containing a color former and a developer. The heat-sensitive recording sheet is imagewise heated by, for example, a heat generation element such as a thermal head to carry out image recording by a thermal color forming reaction of the color former and the developer.
Hitherto, heat-sensitive recording sheeets have been prepared by applying an aqueous dispersion of a mixture of a colorless or light colored color former such as Crystal Violet lactone, etc. and a developer such as a phenolic compound, etc. to a paper or plastic film as the base and drying the applied dispersion to form a heat-sensitive layer. Further, the recording layer often contains various additives such as wax particles, wax emulsions, fatty acid metal salts, fatty acid amides, etc. for the purpose of improving color forming characteristics, pressure-sensitive color forming properties, lubrication properties, etc. or contains white pigments such as clay, talc, titanium oxide, etc. for the purpose of improving whiteness or writability of the surface of recording layer. Such heat-sensitive recording sheets are described in, for example, U.S. Pat. No. 4,265,978.
The foregoing heat-sensitive recording sheets form a color image on the heated part when imagewise heated by a heating element. However, the color image discolors or fades with heat or humidity. Moreover, the image fades after several days even at a room temperature, if touched 2 or 3 times by the fingers. Consequently, it becomes impossible to read the letters. Various attempts for preventing the fading phenomenon have been tried, as described in U.S. Pat. No. 4,255,491. For example, heat-sensitive recording sheets comprising 3-diethylamino-6-methyl-7-anilinofluoran or 3-pyrrolidino-6-methyl-7-anilinofluoran as the color former are known to minimize stains caused by heat or humidity. However, such heat-sensitive recording sheets have other faults in that they have low whiteness and the background thereof is easily fogged.